Secondary recovery by miscible fluid displacement



4 Sheets-Sheet l w. M 6 W w. D n

B. HABERMANN SECONDARY RECOVER/Y BY MISCIBLE FLUID DISPLACEMENT Jan. 26, 1965 Filed INVENTOR. ENZ/0N #45e-RMA /v/v /J rraR/VEY PLY/SOUWLENE REA COVE/QED Jan. 26, 1965 a. HABERMANN 3,167,118

SECONDARY RECOVERY BY MISCIBLE FLUID DISPLACEMENT Filed July 6, 1959 4 Sheets-Sheet 2 MAL/iii EXMPL'Z INVENTOR.

BfA/z/m/ #45E/@MANN rraRA/Ey Jan. 26, 1965 B. HABERMANN 3,167,118

. SECONDARY RECOVERY BY MIscIBLE FLUID DISPLACEMENT Filed July 6, 1959 4 sheets-sheet s l NV EN TOR. BENZ/0N #Agfa/W4 A/A/ [MJ c wat Jan. 26, 1965 B. HABERMANN 3,167,118

SECONDARY RECOVERY BY MISCIBLE F'LUID DISPLACEMENT Filed July 6, 1959 4 Sheets-Sheet 4 INVENTOR. @QA/z/a/x/ A//IBEQA/MA/A/ United States Patent 3,167,118 SECQNDARY RECOVERY BY MISCIBLE FLUlD DHSPLACEMENT vIienzioiu Habermanu, Montebello, Calif., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Filed July 6, 1959, Ser. No. 825,128 Claims. (Cl. 16o- 9) This invention relates to secondary recovery of petroleum from subterranean reservoirs. Specifically, it relates to a miscible fluid displacement method of secondary recovery with an improved areal sweep efficiency.

In attempts to recover petroleum from subterranean reservoirs in addition to that recovered by primary techniques, much attention has recently been focused on the miscible fluid slug displacement method of secondary recovery. In this method, a fluid miscible with both the crude petroleum and a drive fluid is injected Vinto the formation from an injection well and is forced through the formation to drive or carry the crude petroleum towards a suitably located production well. Examples of miscible fluids which have been so employed are L.P.G., propane, butane, pentane, and petroleum distillates. Since such uids are relatively expensive they are in turn forced through the formation by a less expensive driving fluid, e.g., natural gas, methane, ethane and mixtures of natural gas and water. The miscible fluid is then employed in the form of a slug interposed between the driving fluid and the crude petroleum, and is provided only in an amount sutlicient to insure separat-ion of the driving fluid and the petroleum.

Although miscible fluid displacement provides high oil recovery from the swept area, much difficulty has been experienced in obtaining a satisfactory areal sweep of the displacing fluids. The poor areal sweep of the miscible fluid slug has been found to be related to the unfavorable mobility ratio between the miscible fluid and the oil. The mobility ratio is determined by the following equation:

K M=ifl= 2 5 1 #2XL where M=mobility ratio;

a2, la1=viscosity of displacing fluid and displaced fluid,

respectively;

K2, K1=permeability of the formation with respect to the displacing fluid and displaced fluid, respectively.

The areal sweep efficiency is indirectly proportional to the mobility ratio, and, as the above equation shows, the latter is directly proportional to the ratio of displaced fluid viscosity to displacing fluid viscosity and directly proportional to the ratio of displacing fluid permeability to displaced fluid permeability.

In most m-iscible fluid displacement operations, the permeability ratio, Kz/Kl, is more or less fixed by the inherent characteristics of the formation. Thus, the viscosity ratio, ,r1/a2, is the controlling variable in the mobility ratio, and becomes the determining factor in the areal sweep efficiency of a miscible fluid displacement.

O Fice Any reduction of the viscosity ratio, particularly if such ratio is normally greater than 1, will result in a better areal sweep, provided the miscibility of the fluids is not disturbed. Previous attempts to improve the sweep efficiency have centered around attempts to adjust either the miscible fluid viscosity, only, or to adjust the mobility ratio between the drive and miscible fluid phases, only, without compensating for an adverse permeability ratio, and without regard to the -adverse effect such adjustments may have on the miscibility of the drive fluid with the miscible fluid. The reported studies on this problem have also been vconducted at relatively low mobility ratios between the drive and the oil vphases and, therefore, are not indicative of actual reservoir con-v ditions where unfavorable mobility ratios of the order of 30 to 80 are frequently encountered.

This invention is directed to improving the areal sweep efficiency of a miscible fluid displacement under reservoir conditions. It is the purpose of this invent-ion toattain favorable mobility ratios between the drive fluid, the displacement fluid and the reservoir oil while retaining miscibility of the fluids. In reservoirs which have a low permeability with respect to the connate oil due to free gas saturation, it is also within the scope of this invention to compensate for the adverse permeability ratio of oil to miscible fluid by increasing the miscible fluid viscosity to a greater value than the oil viscosity, thereby obtaining the most favorable mobility ratio. It is also a purpose of this invention to retain high solubility of the miscible displacement fluid for the driving fluid. Another aspect of this invention is a method to plug fingers and ow channels in a formation by depositing a polymer in the fingers.

These purposes are achieved in accordance with this invention by the addition of a long chain polymer to the miscible displacement fluid. Small amounts of a long chain polymer dissolved in a light hydrocarbon cause a substantial increase in viscosity of the solution. The presence of the polymer has little effect on the solvent properties of the displacement slug, and the miscibility of the latter with both the crude oil and the driving fluid is relatively unaffected. Under these conditions, high areal sweep efficiencies can be attained. Drive fluids employed are methane, ethane and `natural gas, alone, or mixed with water. In a preferred modification, the viscosity of the drive fluid is adjusted to provide a low mobility ratio between the drive and the miscible fluids. This may be achieved by use of a mixed water-gas drive, by simultaneous injection of gas and water, or gas and an aqueous polymer solution. During displacement, the

'reservoir is kept under sufficient pressure to keep the miscible fluid in a liquid state but not under a pressure suiliicent to cause fracturing, i.e., less than about 3,000 p.s.i.

The invention will be described in greater detail hereafter and illustrated by the figures of which:

FIGURE l illustrates the viscosity of hydrocarbon solutions of long-chain polymers;

FIGURE 2 illustrates a section of a producing field in which the invention can be practiced; f

FIGURE 3 illustrates the amount of oil recoverable as a function of the mobility ratio in a typical oil displacement process;

FIGURES 4 and 5 illustrate the areal flow pattern in the displacement described in Example l;

FIGURES V)6` and 7 illustrate the areal flow pattern of the displacement process described in Example 2;

FIGURES Sand 9 illustrate the areal ow pattern for the displacement process described in Example 3; 3

FIGURES l()V and 11 illustrate the areal flow pattern obtained -in Example 4 when using a displacement process in accordance with my invention;

FIGURE 12 illustrates the miscibilities of voir oil, displacementmedium and drive gas;

FIGURE 13 ,illustrates `the gradation in viscosity of ay plurality .ofviscous displacing` fluids; and

the reser- FIGUREV 14illustrates the processisteps of oner embodiment of my invention,wherebyincreased recoveryl of reservoir. oil is attainable.

. The increase in viscosity effect'noted withns'olutionsV of long chainpolymers isrelated tothe molecular Weight and shaperof the polymer chain. The relationship between viscosity and molecular'weight is'as follows:

Y lYLl=KZlifa where "j K and a are constantsY for a given seriesof polymers;

M equals the polymer .molecular weight;

[n] is the intrinsic viscosity and is obtained by extrapolating tozero concentration a plot, of:

versus concentration, where C=concentration;

n=viscosityofsolution;

n=viscosity of solvent. n

The values of'K and a are reported in available literature Von polymers-, c g., Synthetic Rubber, by G. S.`:Whitby, p.

yAny, long chain oil-Soluble polymer exhibiting a high solution Viscosity is appropriate for use. in accordance. y with the invention.V Examples of suitable polymers are:

polymers of butadiene, isobutylene, chloroprene, .copolymers of butadiene and styrene, methacrylate, or acrylovnitrile, copolymers of isobutylene .and isoprene, Anatural rubber, andV polyvinyl Ychloride .plasticized with4 tricresyl phosphate., The 'above formulas show that the increase in viscosity of a polymer solution is a direct function of the polymer molecular weight. Expressed inanother form, the formulas show that the greater the polymer and displacement fluid will, kof* course, be dictated by reservoir characteristics and economic considerations.

, All of the above mentioned polymers and solutions are illustrative only and are not to limit the invention which`V is operative with any viscous oil-soluble polymer solution which is miscible with both the drive lluid and the reservoir oil. In those cases Where the polymer is insoluble or only slightly soluble in the miscible fluid employed,

vsuch as polyisobutylene which Vis not soluble in hydrocarbons lighter than pentane, it is Within the scope of this invention to add a second uid to the miscible fluid as a secondarysolvent, hereafter referred to as a solutizer,y to

increase "the polymer sol-ubility.r Examples of such solu- 0f oil.

molecular weight,the lower the concentration of the Ypolymer'needed to. achieve any given solutionviscosity.. The

maximum molecularY weight of polymers useful invthis in vention is .limited only* bythe solubility of the polymerV in the miscible displacementuid; Although the lower vmolecular weight'A polymers are more soluble, their use is disadvantageous .since relatively large quantities of the polymer mustbe added to' achieve a satisfactory viscosity increase. The preferred polymers are those which effect a substantial increase in -the viscosity of 'the displacementy fluid at concentration lessthan about 1y percent by weight,

and which have. molecular Weights greater than about 10,000 and preferably between about 500,000 and 1,500,000.Y Sufficient polymer is added to the hydrocarbon liquid to cause more than a 10- percent increasein viscosity. The amount of polymer yadded varies `for eachV polymer'but is generally between 0.01 and A1 percent by weight, and preferably from 0.05 to 0.5 percent by Weight. The displacement fluid can'. be anyliquid-in which-the polymercan be dissolved vand whichjis miscible with the connate oil and the drive fluid;- Examples ofY suitable l fluids include petroleum distillates, crude oil, petroleum` ether, parain hydrocarbons such as butane, pentane,

lhexane, heptane, L.P.G`., aromaticspsuch as xylene, toluene, benzene, coal tar oils,-cyclohe'xane, or mixtures thereof. Preferred uids are LRG., propane,gbutane, isobutane, pentane, isopentane, hexane, isohexane, gasoline and kerosene, The optimum combination of polymer tizers are cyclohexane, decalin, tetralin, pyridine, or anykv of theabove mentioned solventsfor the polymer. The following table shows the effect ofsuch solutizers on the solubility of polyisobutylene vin isobutanez;

. VThe effect of polyisobutylene addition on the viscosity of normal heptane is` shown in FIGURE 1, which is-a Y plot1of`vis'cosity in centipoises of the solutionv versus.-

weight percent of f polyisobutylenein normal heptane.y

From this' graph, it is seen that polyisobutylene in as lowA 'a concentration as 0.3 weight percent will more` than quadruplethe viscosity of normal heptane froma .value1 ofk 0.4 centipoise toy 1.68 centipoises at F VThissubstantial increase in solution Yviscosity lforlow concentrations of polymer isa characteristic, ofzalfthe aboveimeni tioned long chain polymers kand is an Himportant'factor.

in practicing the invention since it affords an economical method Vof increasing Ythe viscosity of anioil-miscible fluid to the degree necessary for use in the secondary recovery- It is of course apparent that other oil-miscible fluids and other solutizers may be used, e.g., L.P.G.', as

a miscible fluid with added hydrocarbon solutizers suchr as petroleum distillates or even a portion of the crude oil. solutizer has the advantage that the viscosity ofjthe solutizer has an additive effect on thersolution viscosity. The choice of any'oil-miscible Vfluid and solutizer will be governed largely by economioqconditions and the avail-4 ability ofthe components.v y K As previously mentioned, the,V driving fluid maycomprise an aqueous polymer solution which is injected into the formation simultaneously with a gas such as methaneV Preferably, the polymer isxa,

ethane or reservoir gas. water-soluble. partially hydrolyzedl acrylamide polymer. These partially hydrolyzed acrylamide Vpo1ymersare water-solubler acrylamide polymers Whichhave-beenV hy-V drolyzed to such VVan Yextentthat between-about 0.8.and 10 percent of the amide groups have Abeen converted to car-y boxyl groups. As herein employed, the term acrylamide polymer is inclusive of the homopolymersof acrylamide, i.e., polyacrylamide, and water-soluble *copolymersy of acrylamide'with up to about 15 percentbyweight of otherpolymerizablevinyl compounds, such as the alkyl esters. of acrylic and methacrylic acids, methacrylamide, styrene, vinyl acetate, acrylonitrile, methacrylonitrile, vinyl alkyl ethers, vinyl chloride, vinylidene'chloride, etc.V In addi-f y tion yto the "aforementioned". limitation on the 'extent off hydrolysis, the acrylamide polymers suitable arevof'suiii-v Y ciently high molecular `weight that af 0.5 percent by weight aqueous-solution thereof has a viscosity of at least" The use; of a viscousdistillate or crude oil asaV 4 centipoises Ostwald at 21.5 C. A commercially available polymer meeting these requirements is marketed by The Dow Chemical Company under the name Separanf The above mentioned polymers are preferred, since they form a stable solution and do not easily precipitate when heated or when in the presence of mineral anions and cations, however, the use of any other water-soluble material which forms a stable viscous aqueous solution is also within the scope of the present invention; examples of such materials include polyvinyl alcohol, carboxymethyl cellulose, sucrose, etc.

The use of an oil-soluble polymer dissolved in a light hydrocarbon as the miscible fluid slug permits adjustment of the viscosity of the miscible displacement fluid through a range of values from that of' the low viscosity light hydrocarbon solvent up to and exceeding the viscosity of the reservoir toil, thereby permitting compensation for adverse permeability ratios. It is also possible to stage the solution viscosity at successively decreasing or increasing values from the miscible fluid-oil interface to the miscible fluid-drive fluid interface without laltering the miscibility of the fluids. This permits gradual change from the high viscosity oil to a low viscosity drive without any sharp change in viscosities,` thereby avoiding high mobility ratios. Conversely, it is possible to gradually increase the viscosity from a low viscosity oil to a high viscosity drive.

Adverse permeability ratios may be compensated for in accordance with the following discussion. The mobility ratio, M, of the fluids is directly proportional to the ratio of the permeability with respect to the miscible slug, KS, to the permeability with respect to the connate oil, KO. Under reservoir conditions, the ratio Ks/Ko is generally greater than one. Ko is lfrequently relatively low by `reason of the presence of free gas saturation of the oil phase occurring when the reservoir pressure is below the bubble point pressure, which is defined as the highest pressure at any given temperature at which gas bubbles will form in the oil phase. This condition can occur naturally or be caused by pressure depletion during primary recovery. Ks, however, is not reduced proportionately since the mixed oil and gas phase is swept out of the formation ahead of the slug. As a consequence, the permeability ratio is adverse, and even though the miscible slug viscosity is increased to equal the oil viscosity, the mobility ratio of the fluids, M, will not lbe corrected for the adverse permeability ratio. However, by practicing this invention in the use of a polymer additive to the miscible fluid slug, the vslug viscosity may exceed the oil viscosity, and the most favorable mobility be achieved by setting the slug viscosity in accordance with the following equation:

Ks Ms: #UE

In addition to the above adjustments of mobility ratios possible through use of polymer solutions, another unique procedure may be followed after fingering has'occurred and fthe miscible fluid slug has broken through to the producing well. This procedure is illustrated in FIGURE 14. As illustrated, the aforedescribed miscible displacement is continued until breakthrough, steps l to 3. Upon breakthrough, step 4, the pressure on the well is reduced, whereby the displacement fluid vaporizes and the dissolved polymer precipitates. During depressurization, there will be `a flow of the polymer solutions into the finger or flow channel, concentratin-g the polymer in this flow channel where it will :deposit and seal off the channel. When this has occurred, the reservoir is again pressurized and the miscible fluid drive repeated. The polymer deposited in the fingers of the preceding drive will remain in place, sealing these flow channels, and a new flow pattern will occur with the second drive, thereby increasing the total area swept. This procedure can be improved by use of methods to detect the miscible fluid slug before it reaches the producing well. When proximity of the slug to the carefully chosen, however, so as not to affect the vertical and horizontal sweep of the miscible fluid slug.

In some instances, it may not be necessary to reduce the reservoir pressure and vaporize the solvent to deposit the polymer. The mixing between a drive fluid and a miscible polymer slug is greater in a nger or flow channel than with a smoothly advancing front. If the polymer is not soluble in the drive fluid, this mixing of drive fluid and polymer slug will cause the polymer to precipitate selectively in the finger and thereby even the flow pattern.

Referring now to FIGURE 2, a diagram of a producing field is shown. The shaded area A within the dashed lines is a representative portion of this field and with its injection well, I, and production well, P, is the area chosen for study in FIGURES 4 to 11. In a series of laboratory studies, area A was simulated by a consolidated sandstone plate one-eighth or one-fourth inch thick by fifteen inches square. between two plates of L11-cite. Inlet and outlet holes corresponding to inlet well I and production well P, were provided at opposite corners of the assembly, and suitable connections permitted liquids to be forced into the sandstone plate and through the same towards `the outlet connection. This assembly was supported in a horizontal plane above a light source, and a camera was positioned directly above the assembly to photograph the flow of colored fluids as they were passed from the inlet to the outlet connection. Cooled air was blown across the bottom surface of the assembly to prevent the hot light source from heating the assembly during a test run. Color slides or color movies were made during the tests to evaluate the flow patterns, and the outlet flow was measured to determine the amount of oil recovered. By this method a continuous record of the flow pattern was obtained. These photographs are depicted by FIGURES 4 to ll for varied flow conditions. The fluids chosen for the flow studies were diethylene glycol, isopropanol, hexane, light mineral oil, carbon tetrachloride, and solutions of polyisobutylene in hexane and carbon tetrachloride. Vertical sweep difficulties were yeliminated by blending the fluids to match densities and by use of t-hin sandstone composites, one-eighth or one-fourth inch in thickness. The above fluids were blended to obtain the desired viscosities and densities for each test run. All fluids had approximately equal permeabilities in the sandstone composite. The mobility ratio between any two fluids was thus equal to their viscosity ratio. A two phase systern was first studied and the results of this study are shown graphically in FIGURE 3, which is a plot of percent recovery of oil in place at breakthrough versus the mobility or viscosity ratio. Also plotted in FIGURE 3 are the percent of area swept, determined by planimetering the enlarged color slides of the flow pattern at break` through. Breakthrough is defined as the point at which the displacing fluid phase reaches the output well or connection. Some additional recovery of oil will be achieved after breakthrough by continued displacement, but the displacing fluid soon channels down the finger and no longer displaces any appreciable amount of the oil in place. Thus, breakthrough recovery data give good indications as to the efliciency of the process. An inspection of FIG- URE 3 shows that the area swept at any mobility 1"atio is greater than the percent recovery of oil in place. This is because the oil and displacing fluid mix at the interface and some of the oil in place dissolves in the miscible displacement fluid. FIGURE 3 thus gives an indication of In each experiment the plate was sealedv the amount of miscible huid-necessary to maintain sep? aration of thedrive iiuid lwith the oil phase. At a mobility ratioof. 10,-the mixing Zone, lor difference between area covered and percent oil recovered, amounts to Vabout 9% of .the-porevolume Therefore-.the misciblev iiuid should g be employed in-an amount corresponding to 9% of the reservoirpore Volume in'order to lmaintain separation of the drive fluidi and theoilphase;` 'FIGURE 3 further shows Y tha'tfthe. size of f the mixing zone, or amount -of oil ,dis-

v solved inthe displacement uid, decreases as the mobility ratio between thelfluids is decreased.k The decreased volurne of mixingzzone with Ydecreased mobility-ratio relationship thus justifies the use of the more expensive hydrocarbons, eg., pentane, as miscible displacement fluids.

For example, under reservoirconditions, the viscosity ratio. of crude oil to propane is about 16. Polyisobutylene can be dissolved in p-entanefto givea mobility ratio of 1" vorl less. The sizie of the mixing zones asvdetermined by FIGURES is 0.09 pore volumeforpropane and 0.04 pore volume for r the pentane solution.' Therefore, although pentane is more expensive than propane, the lesser-amount of pentane than propane lrequired for displacement will olfset thelhigher unit cost of pentane. Inaddition, the oil Simulated oil to slug 1 l 3.0 Slug 1 to slug 2 2.9 Y Slug 2 to slug 3 V3.0 Slug 3 todrve fluid 2.9 Simulated Aoil to drive fluid 75.0

recovery has been increased from toV 66.4% of oil in place- FIGURE 3 also indicatespthat the amount-of miscible fluid-employed should be greater than 0.03 pore tions as Ythe slug-drive interface is approached to equalize the viscosities at the slug-drive interface. Such a system would-have complete miseibility between the drive gas and all-portions of the-slug phase. A mixture of gas and aqueous'polymerv solution mightlalso be employed as the` drive iiuid to achieve a viscousdrive in combination with the viscousjmiscible uid slug. Again, the .isobutylene concentration may be staged between. the oil interface vandthe drive interface to grade evenly the'viscosity difference between the oil and drive phases.

The following examples will serve to'illustrate the invention: f

YExample 1 FIGURES 4 and 5 depict the flow pattern of a run with a viscous miscible fluid 'slug of 10 percent of pore volume followed by a low viscosity drive. Ther sandstone formation was saturatedwith'a simulated oil consisting ofY a mixture of diethylene glycol and isopropanol. Isopropanol was employed -as the rniscibleV displacement iiuid, and a mixture of C6 hydrocarbon was employed as the driveY iiuid. The -miscible slug `was miscible with both the oil fand the drive fluid, but the latter `was imrniscible n with the foilff The viscosity ratios were:

- Simulated oil to miscibleuid 11.3 yMiscible lluid to drive fluid 6.63 Simulated oil to drive iluid 75.0

' Theflow pattern at 20 percent pore volume of tiuids injected is shown in FIGURE 4. The iiow pattern at breakthrough is shown in FIGURES. The recovered oil at breakthroughwas 27-.6 percent of the oil in place. FIG- URE 5 shows pronouncedfingering of the displacement fluid and a relatively poor areal sweep eiciency due to the lhighlmobility ratios encountered. The nalbreakthrough was'achieved lby immiscible. iiow through the oil phase, representedxby the dashed V`linesuextending from the tip of the nger tothe producing well. Thisflow was due vtolthe drive fluid which .broke through the miscible fluid slug phase and -passed'into the oil phase. When this" happens, the drive iluidy channels `directly to the producing where the areal sweep is shown at 20 percent recovery of well in Yvery thin well defined paths and efficient areal sweep is flost.' This breakthrough..as well as the pro- Vemployed werer' Y nounced iingering,.was due to thehigh mobility ratios.v

employed. f ExampleZ .y

FIGURES 6 and 7 depicrfanotae.- now system with. the sameratio of oil todrive tiuid viscositiesyhowever,in this test, the miscible fluid slug was gradated into three bands of4 successively lower viscosities-from the oil interface to the drive interface. `vThe viscositygradients ofthe slug were obtained-bymixingkdiethylene glycolwith isopropanol for slug portions 1- and'2 vand mixing 'hexane .with isopropanol lfor slugs. All other'conditions were duplicates of those employed in Example 1. The'viscosity ratios i The ow pattern after all three slugs had been injected isY shown in FIGURE 6 at 25 percentpore volume of luids injected. InV FIGURE '7, theilow pattern is shown whenY breakthrough occurred. The recovered oil at breakthrough `was 25.5 percent of the oil in place.

The high viscesity-ratio `of between the dra/enum o and the simulated oil phaseis not an uncommon gas driving condition in many reservoirs. If the driving gas has a viscosity-ofr0-0l-7 centipoise at reservoir conditions of 260 F. and 2100 p.s.i., the viscosity of the crude in the reservoir would only have to be 1.28 centipoises, a value.

often exceeded with heavy crude.

o ExampleS l To revaluate the effect of -a viscous drive with a low viscosity -miscible fluid ',slug, a test was made displacingV light mineral oil from the sandstone layer with a miscible slug consisting of a mixture of hexane and carbon tetra-v chloride followed by a mineraloil drive. n Theviscosity ratios were: Y y n V Oil to slug 38.0 Slug to drive 0.265

Oil to drive 1.0 rEhe oil recovery rat breakthrough was 23.6 percent of the oil in place. The flow patterns at 20 percent pore volume of vfluid injected fand atf breakthrough are shown by FIGURES 8 and 9, respectively. The recovery in this system was no better than obtained in Examples 1 and 2. FGURES 8 and 9 show extreme fingering of the lslug phase through the oil with poor areal sweep oflthe miscible slug. Continued displacement'in this case willshowa relatively high areal sweep of the drive fluid because the mobility ratio between theoil and drive is equal to 1. f However, the miscible slug fingers through ahead of the drive and is not retained between the drive and 'oil phases.

This tendency, whichV is shown at breakthrough in the rightside-of FIGURE 9, Aisincreased with continued ydisplace-k ment.l Once the rmiscible slug is vlost from between the immiscible oil and dn've phases, substantial amountsof oil will be by-passed and left behind as residual oil in the. swept area, because the drive luid usually employed is immiscible withthe oil phase.

Example 4 This example demonstrates the .preferred` form of the invention. To evaluate the eifect of a viscous'miscible slug with a viscous drive, Example 3 was repeated, but polyisobutylenel'was added to the miscible slug ina concentra-v tion of- 0.62gram per fcc. of Slug, so as to raise the slug.

viscosity to approximately that of the v'mineral oil. The results of this test areshown in FIGURES 10 and 11,

the oil in place and at breakthrough, corresponding to 60.4 percent recovery of the oil vin place. VFIGURE 1l also `shows that although ingering offthe Slug phase occurred, it did not break through the oil phase until the oil recovery was almost four times greater than in Example 3.

An additional improvement of the areal `sweep is shown in FIGURE l1 by the continuous band of slug which remained between the drive and the oil phases. This is important, since under reservoir conditions the drive and oil phases are immiscible with each other and must be separated by a miscibleslug to assure that oil is not bypassed and left behind in the reservoir.

The above examples show that use of a Viscous drive with a viscous miscible slug achieves substantial improvements over either the use of a viscous slug with a low viscosity drive or a low viscosity slug with a viscous drive. Use of a low viscosity drive with a gradated viscosity slug shows no improvement in oil recovery at breakthrough over that of a non-gradated slug, although there was a slightly better areal distribution of the gradated slug than the non-gradated. Because of this better distribution, continued displacement after breakthrough would show a greater recovery of oil in place than would continued displacement with a non-gradated slug.

Under reservoir conditions with a methane drive, the use of a miscible iluid slug made viscous by blending with crude oil or distillates poses a problem of immiscibility of the slug and drive phases. This is apparent from a study of FIGURE l2, which is a phase diagram for a methane, butane and Cq-jsystem. This system was worked out for a reservoir at 260 F. and under 2,000 p.s.i. The viscosity of the drive gas was 0.017 centipoise and that of the oil phase was 0.66 centipoise. The miscible fluid, butane, was divided into three slugs blended with reservoir oil to give equal viscosity ratios between each portion, from pure butane with a viscosity of 0.09 centipoise at the drive interface to a viscosity of 0.34 centipoise at the oil interface. The viscosities of the slugs are shown on FIG- URE 13. The compositions of the slugs necessary to provide these viscosities are shown on FIGURE 12. Slug 1, in contact with the oil, contains 69 percent oil. The next slug contains 35 percent oil, and the last miscible slug is pure butane. This pure butane slug is followed by a reservoir gas drive. During the displacement, lingering will occur, because the mobility ratios are not equal to l, and the gas drive penetrates into the slugs` of butane and oil. When the gas reaches slug 2, in a concentration greater than 29 percent, the slug and gas become immiscible and a large portion of the gas will flow directly to the producing well in very narrow channels. As a result, the efficiency of the miscible slug has been greatly reduced.

The limiting concentration of oil in butane to assure complete miscibility of the gas drive with the slug is shown by A on FIGURE l2 to be 95 percent butane and 5 percent oil. With such a low concentration of oil, no substantial increase in slug viscosity over that of butane can be obtained. This demonstrates that it is not possible to obtain aviscosity graded slug and still retain miscibility of the gas drive and all of the slug under reservoir conditions by blending oil stocks in the slug to increase the slug viscosity. Miscibility and viscosity increase can be obtained while retaining miscibility throughout the slug with a gas drive, however, by practicing this invention and adding a polymer to the miscible slug to increase its viscosity.

The examples have been given by way of illustration only and are not to be construed to limit the scope of this invention which is applicable to the use of any oil-soluble long chain polymer in an oil-miscible solution as a slug for a miscible luid displacement method of secondary recovery.

I, therefore, particularly point out and distinctly claim as my invention:

l. In the miscible fluid displacement of oil from a subterranean reservoir penetrated by at least one injection well and at least one withdrawal well wherein a body of hydrocarbon liquid which is miscible in said reservoir with said oil is injected into said reservoir through said injection well and forced to sweep through said reservoir by the subsequent injection of a drive gas through said injection well, thereby displacing said oil from said reservoir and forcing the displaced oil from said reservoir through said withdrawal well, the improved method of obtaining a high areal sweep eiiciency while retaining miscibility between said gas drive and said hydrocarbon liquid which comprises dissolving a long chain oil soluble polymer having a molecular weight of at least 10,000 in said hydrowhere as, ,to are the absolute viscosities of the hydrocarbon-polymer solution and said oil, respectively, and KS, KD are the permeabilities of said reservoir to said hydrocarbon-polymer solution and to said oil, respectively.

3. The method of claim 1 wherein said polymer is polyisobutylene, said hydrocarbon liquid comprises one or more members of the group consisting of butane, isobutane, L.P.G., and propane, with a solutizer, and said gas drive comprises one or more members chosen from the group consisting of natural gas, methane and ethane.

4. The method of recovering oil from a subterranean reservoir which comprises pressurizing said reservoir by injecting into a rst well communicating with said reservoir at least 3 percent of said reservoir volume of a liquid hydrocarbon solution of an oil-soluble long chain polymer with a molecular weight greater than 10,000, thereafter injecting a drive gas into said reservoir, maintaining sufcient pressure on said reservoir to retain the injected hydrocarbon in liquid phase within said reservoir 'by controlled withdrawal of displaced oil from said reservoir through a second well communicating with said reservoir, detecting the proximity of said liquid hydrocarbon to said second well, reducing the pressure on said reservoir when said liquid hydrocarbon-polymer solution reaches said second well, withdrawing vaporized hydrocarbon, repeating said pressurization by repeated injection of said liquid hydrocarbon solution of said polymer followed by repeated injection of said gas drive, recovering additional displaced reservoir oil, and repeating said depressurization of said reservoir when said liquid hydrocarbon solution of said polymer again reaches said second well, continuing said repetition of the pressurization and depressurization steps until said reservoir is substantially swept free of all oil in place.

5. The method of recovering oil from a subterranean reservoir which comprises pressurizing said reservoir by injecting a liquid hydrocarbon solution of an oil-soluble long chain polymer having a molecular weight greater than about 10,000 into said reservoir through a first Well communicating with said reservoir, forcing said liquid hydrocarbon solution through said reservoir by the subsequent injection of a drive gas into said irst well, maintaining suicient pressure on said reservoir to retain the injected hydrocarbon in liquid phase within said reservoir by the controlled withdrawal of displaced oil from said reservoir through a second well communicating with said reservoir until said liquid hydrocarbon solution reaches said second well, thereafter reducing the pressure on said reservoir to below the vapor pressure of said liquid hydrocarbon within said reservoir to thereby vaporize said liquid hydrocarbon and deposit said dissolved polymer Within the iiow channels of said reservoir, withdrawing vaporized hydrocarbon, repeating said pressurization by repeated injection of said liquid hydrocarbon solution of Vsaid polymer` followedby repeated injection of said gas drive, recovering kaddi-tional `displaced reservoir oil, repeating saidV depressurizationof said reservoirfwhen said liquid hyrlrocarbon solution again reachesjs'aid second well, continuing' said repetition vof the'pressurization and depressurization steps until said reservoir isfsubstantially swept free of oil.

Y vReferences Cited-in the iile of this patent UNITED STATES vPATENTS Y 2,341,500 r Darling 1 Feb. 8, 1944 

1. IN THE MISCIBLE FLUID DISPLACEMENT OF OIL FROM A SUBTERRANEAN RESERVOIR PENETRATED BY AT LEAST ONE INJECTION WELL AND AT LEAST ONE WITHDRAWAL WELL WHEREIN A BODY OF HYDROCARBON LIQUID WHICH IS MISCIBLE IN SAID RESERVOIR WITH SAID OIL IS INJECTED INTO SAID RESERVOIR THROUGH SAID INJECTION WELL AND FORCED TO SWEEP THROUGH SAID RESERVOIR BY THE SUBSEQUENT INJECTION OF A DRIVE GAS THROUGH SAID INJECTION WELL, THEREBY DISPLACING SAID OIL FROM SAID RESERVOIR AND FORCING THE DISPLACED OIL FROM SAID RESERVOIR THROUGH SAID WITHDRAWAL WELL, THE IMPROVED METHOD OF OBTAINING A HIGH AREAL SWEEP EFFICIENCY WHILE RETAINING MISCIBILITY 